The entire Mustang line was slated to be called Apache originally. The P51 was a really good airplane below 12k and it still flew above that it just wasnt superb (the British claimed excellent) until the Merlin was mated to it. So the American P51 (without a subletter designation) with four 20mm cannon was at that point still the Apache. The attack version (A36) was called Apache from the factory but the Army never liked that name and suggested a change (service men going with Intruder) but the P51 was never produced in numbers and with the addition of the Merlin all P51As (Allison engine I believe) Bs Cs Ds and so on were called Mustangs.
The Mustang IA was called Mustang by the RAF. It would have been confusing anyway if Americans called them Apaches and the British referred to them as Mustangs.
The Apache was not a bad performer but due to the way they attacked originally (fresh out of training) AAA had no trouble picking them off. The attack procedure was adjusted and losses declined but the Apache was best suited for Armor support (attacking in close support of armor) while also providing front line air superiority and this was unacceptable to the brass that preferred the USAAF find its own glory. The German equivalent would have been the Stuka and for Russia the Il2.
